Studies of a normally ocurring plasmid genome, the mitochondrial genome, in human brain tissues have revealed that most of the mitochondrial DNA is in an oligomeric form. These oligomers appear to represent dimeric, trimeric, and tetrameric forms of the mitochondrial DNA. Previous studies of the mitochondrial genome in human tissues failed to reveal the predominance of oligomeric forms, since they did not involve human postmitotic tissues, such as the brain tissues examined in the current investigations. Lack of mtDNA repair mechanisms may account for the relatively high mutation rate found in this genome, as compared to the nuclear or chromosomal genome. A survey of the clinical literature revealed a number of diseases that display non-Mendelian maternal inheritance patterns and may involve the mitochondrial genome. Studies on the primary structure of mitochondrial DNA from normal individuals and from patients with maternally inherited diseases have been initiated. These studies are being performed in collaboration with the FBI Forensic Science Research Group to determine whether these genetically inherited variations in the human mitochondrial genome are sufficient to establish individuality for forensic studies.